Update the method used to detect if a timer is active. Previously the timer was...

Update the method used to detect if a timer is active.  Previously the timer was deemed to be inactive if it was not referenced from a list.  However, when a timer is updated it is temporarily removed from, then re-added to a list, so now the timer's active status is stored separately.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2589 1d2547de-c912-0410-9cb9-b8ca96c0e9e2